The present invention relates generally to imaging systems utilizing imaging sensors and more particularly to imaging systems with capability of periodically re-setting photosites and requiring a wide dynamic range within the same scene. Field of applications may cover a broad range of areas including but not limited to inspection and testing where imaging is used, digital x-ray systems, surveillance imaging applications, film scanning, night vision and automotive applications.
Typical scenes viewed by a camera may have a wide range of illumination conditions across the image. That is, the scene may have details in dimly lit areas that need to be resolved while simultaneously needing to resolve very bright areas of the scene without saturating the image sensor and camera system. Conventional film-based cameras are able to resolve the detail in both the dimly lit areas of a scene and brightly lit areas of the scene simultaneously because of the non-linear response of film emulsions. Digital cameras, however, have a highly linear response to light. This is both advantageous and disadvantageous. Linear response of a digital camera is frequently desirable in scientific imaging applications because post processing algorithms assume a linear response to light, but the range of information in a scene having very dimly lit and very brightly lit areas may exceed the linear range of the image sensor chip itself or the analog-to-digital converter (A/D) that is used to convert the information into computer readable information. If the gain of the sensor is reduced to avoid saturation in the high brightness area, details can be lost in the dimly lit areas. Conversely, if the gain is increased to capture the details in the dimly lit areas, the bright areas of the image will saturate, and image content is lost.
To achieve performance similar to that of a non-linear imaging device, such as a film-based camera, the linear nature of a digital imaging device thus must be transformed to a non-linear one. The transformation curve, or transfer function, that can correct the linear nature of a digital device closely follows an exponential function. The exponential value has been denoted by the symbol γ by the imaging industry, and hence, the transformation curve is often referred to as the “gamma correction.”
Typically, improved image display is achieved by post-processing the image after it is captured. Post-processing methods usually increase the gain in the dark areas (dim lighting) and decrease the gain in the bright areas, while applying a medium gain to areas between the two extremes. One common method uses a software gamma correction after the A/D converter converts the analog to digital data. While this increases the gain of the displayed image at low light levels, it also reduces the signal-to-noise at the low light levels. Because shot noise at higher illumination makes very fine gray-scale resolution less usable, this method compresses bright light levels and sacrifices information. For example, using this method, an imaging system with a 12-bit A/D converter and a 12-bit sensor may, in practice, generate only 10 bits of useful data.
In a second common method, an automatic gain control (AGC) circuit is included prior to the A/D converter. While this method is useful for shifting between bright scenes and dim scenes, it cannot effectively handle a scene with both bright and dim areas simultaneously.
A third method manipulates the pixel outputs toward a logarithmic response. While this extends the dynamic range, the signal-to-noise (SNR) level at any particular point in the response curve remains limited, since the number of photons captured at each light level is fundamentally unchanged. Improvement of SNR requires an increase in the number of photons captured. Further, because this method requires adjustments on a pixel-to-pixel basis, objectionable artifacts may arise due to pixel-to-pixel non-linearity. More specifically, such non-linearities vary from pixel-to-pixel because of device fabrication variations, for example, implant variations, oxide thickness variations, variations in bias line resistances, and the like, and/or because of the operation of the pixel transistor in a non-linear region.
A fourth method first captures an image using a short integration time, and a second image using a relatively longer integration. The two images are subsequently combined with the aid of software that takes the bright data from the image taken using a short integration image, and the dim data taken using a relatively longer integration image. This method takes longer to capture an image, and the procedure to effectively combine the two images is difficult and error-prone.
FIG. 1A shows two pixels 10 and 12 of a typical CCD 100. Each pixel is a metal oxide semiconductor (MOS) structure that includes, in part, a conductive doped polysilicon layer 116 formed over a silicon substrate 118 and separated from the substrate by an insulating material 114, such as silicon dioxide. A storage area, or a potential well 108 (to use a water analogy), is formed when a voltage 106 is applied to an electrode 124 deposited over insulating material 114. As light strikes a CCD pixel (sensor), the impinging photons create electron-hole pairs, and the created electrons 112 are stored in well 108. Electrons 112 are confined under electrode 124 within the potential well 108 having barrier height 120. In the example of FIG. 1A, Pixels 10 and 12 are shown as being exposed to the same light density.
Referring to FIG. 1B, pixel 10 is shown as being exposed to a relatively brighter light than pixel 12. Accordingly, after well 108 fills up with electrons, excess electrons 132 begin to spill into adjacent well 134 of pixel 12. The spillover of the excess electrons results in a white streak in the image, and is referred to as blooming.
FIG. 1C shows a pixel 150 with an anti-blooming drain 152, as known in the prior art, adapted to alleviate the spillover problems associated with pixels 10 and 12 of FIG. 1A. The anti-blooming drain 152 is used to from a well 110 whose depth 122 (barrier height) is controlled via a voltage 102 applied to drain 152. The top 154 of well 108 is defined by the voltage 104 applied to gate electrode 126 which is disposed between the pixel electrode 124 and the anti-blooming drain electrode 152. In normal operation, anti-blooming well 110 captures the excess electrons 132 thus inhibiting excess electrons 132 from spilling over to an adjacent pixel. The voltage 102 applied to anti-bloom drain is typically set to the value at which the pixel saturates with charge.